This invention relates to a photosensor system having a two-dimensionally arranged photosensor array, and a method for drive-controlling the system.
Imaging apparatuses such as electronic still cameras, video cameras, etc. have come to be very widely used. These imaging apparatuses employ a solid-state imaging device, such as a CCD (Charge Coupled Device), which serves as a photoelectric converting device for converting an image of a to-be-photographed subject into an image signal. As well known, the CCD has a structure in which photosensors (light receiving elements) such as photodiodes, or thin film transistors (TFT: Thin Film Transistor) are arranged in a matrix, and the amount of pairs of electrons and positive holes (the amount of charge) generated corresponding to the amount of light entering the light receiving section of each sensor is detected by a horizontal scanning circuit and a vertical scanning circuit to thereby detect the luminance of radiation.
In a photosensor system using such a CCD, it is usually necessary to respectively provide scanned photosensors with selective transistors for causing the scanned photosensor to assume a selected state. In place of the combination of the photosensor and the selective transistor, a photosensor (hereinafter referred to as a “double-gate photosensor”) is now being developed, which is formed of a thin film transistor having a so-called double-gate structure and has both a photosensing function and a selecting function.
FIG. 16A is a sectional view illustrating the structure of such a double-gate photosensor 10. The double-gate photosensor 10 comprises a semiconductor thin film 11 formed of amorphous silicon, n+-silicon layers 17 and 18, a source electrode 12 and a drain electrode 13 formed on the n+-silicon layers 17 and 18, respectively, a top gate electrode 21 formed above the semiconductor thin film 11 with a block insulating film 14 and an upper gate insulating film 15 interposed therebetween, a protective insulating film 20 provided on the top gate electrode 21, and a bottom gate electrode 22 provided below the semiconductor thin film 11 with a lower gate insulating film 16 interposed therebetween. The double-gate photosensor 10 is provided on a transparent insulating substrate 19 formed of, for example, glass.
In other words, the double-gate photosensor 10 includes an upper MOS transistor constituted of the semiconductor thin film 11, the source electrode 12, the drain electrode 13 and the top gate electrode 21, and a lower MOS transistor constituted of the semiconductor thin film 11, the source electrode 12, the drain electrode 13 and the bottom gate electrode 22. As is indicated by the equivalent circuit of FIG. 16B, the double-gate photosensor 10 is considered to include two MOS transistors having a common channel region formed of the semiconductor thin film 11, TG (a Top Gate Terminal), BG (a Bottom Gate Terminal), S (a Source Terminal) and D (a Drain Terminal).
The protective insulating film 20, the top gate electrode 21, the upper gate insulating film 15, the block insulating film 14 and the lower gate insulating film 16 are all formed of a material having a high transmittance of visible light for activating the semiconductor thin film 11. Light entering the sensor from the top gate electrode 21 side passes through the top gate electrode 21, the upper gate insulating film 15 and the block insulating film 14, and then enters the semiconductor thin film 11, thereby generating and accumulating charges (positive holes) in the channel region.
FIG. 17 is a schematic view illustrating a photosensor system formed of two-dimensionally arranged double-gate photosensors 10. As shown in FIG. 17, the photosensor system comprises a sensor array 100 that is constituted of a large number of double-gate photosensors 10 arranged in a matrix of (n×m), top gate lines 101 that connect the top gate terminals TG of the double-gate photosensors 10 in a row direction, bottom gate lines 102 that connect the bottom gate terminals BG of the photosensors 10 in a row direction, a top gate driver 111 and a bottom gate driver 112 connected to the top gate lines 101 and the bottom gate lines 102, respectively, data lines 103 that connect the drain terminals of the double-gate photosensors 10 in a column direction, and an output circuit section 113 connected to the data lines 103.
In FIG. 17, φtg and φbg represent control signals for generating a reset pulse signal φTi and a readout pulse signal φBi, respectively, which will be described later, and φpg represents a pre-charge pulse signal for controlling the point in time at which a pre-charge voltage Vpg is applied.
In the above-described structure, as described later, the photosensing function is realized by applying a predetermined voltage to the top gate terminals TG from the top gate driver 111, while the readout function is realized by applying a predetermined voltage to the bottom gate terminals BG from the bottom gate driver 112, then sending the output voltage of the photosensors 10 to the output circuit section 113 via the data lines 103, and outputting a serial data Vout.
FIGS. 18A-18D are timing charts illustrating a method for drive-controlling the photosensor system, and indicating a detecting operation period (an i-th row processing cycle) at the i-th row of the sensor array 100. First, a high level pulse voltage (a reset pulse signal; Vtg =+15 V, for example) φTi as shown in FIG. 18A is applied to the top gate line 101 of the i-th row, and during a reset period Treset, a resetting operation for discharging the double-gate photosensors 10 of the i-th row is executed.
Subsequently, a bias voltage φTi of a low level (e.g. Vtg=−15 V) is applied to the top gate line 101 of the i-th row, thereby finishing the resetting period Treset and starting a charge accumulating period Ta in which the channel region is charged. During the charge accumulating period Ta, charge (positive holes) corresponding to the amount of light entering each sensor from the top gate electrode side are accumulated in the channel region.
Then, a pre-charge pulse signal φpg shown in FIG. 18C and having a pre-charge voltage Vpg is applied to the data lines 103 during the charge accumulating period Ta, and after a pre-charge period Tprch for making the drain electrodes 13 keep a charge, a bias voltage (a readout pulse signal φBi) of a high level (e.g. Vbg=+10 V) shown in FIG. 18B is applied to the bottom gate line 102 of the i-th row. At this time, the double-gate photosensors 10 of the i-th row are turned on to thereby start a readout period Tread.
During the readout period Tread, the charge accumulated in the channel region serves to moderate a low level voltage (e.g. Vtg=−15 V) of an opposite polarity applied to each top gate terminal TG. Therefore, an n-type channel is formed by the voltage Vbg at each bottom gate terminal BG, whereby the voltage VD at the data lines 103 gradually reduces, in accordance with the drain current, with lapse of time after the pre-charge voltage Vpg is applied. More specifically, the tendency of change in the voltage VD at the data lines 103 depends upon the charge accumulating period Ta and the amount of received light. As shown in FIG. 18D, the voltage VD tends to gradually reduce when the incident light is dark, i.e. a small amount of light is received, and hence only a small amount of charge is accumulated, whereas it tends to suddenly reduce when the incident light is bright, i.e. a large amount of light is received, and hence a large amount of charge is accumulated. From this, it is understood that the amount of radiation can be calculated by detecting the voltage VD at the data lines 103 a predetermined period after the start of the readout period Tread, or by detecting a period required until the voltage VD reaches a predetermined threshold voltage.
Image reading is performed by sequentially executing the above-described drive-control for each line of the sensor array 100, by executing the control for each line in a parallel manner at different time points at which the driving pulses do not overlap.
Although the case of using the double-gate photosensor has been described above, even in a photosensor system using a photodiode or a phototransistor as a photosensor, sequential operations of “resetting operation→charge accumulating operation pre-charge operation→reading operation” are executed, and similar control is also executed.
The conventional photosensor systems as above have the following problems.
(1) In an image reading operation employed in the above-described conventional photosensor system drive-control method, when using, for example, the above-described double-gate photosensor as a photosensor, a series of operations are repeated which include application of a reset pulse signal to the top gate terminal TG, application of a pre-charge pulse signal to the drain terminal and application of a readout pulse signal to the bottom gate terminal BG. In this case, each pulse signal has a short pulse wave to be generated for a short time. For example, a high level voltage (e.g. +15 V) is applied for a short time to the top gate terminal TG, and a low level voltage (e.g. −15 V) is applied thereto during the other period. Thus, during the operation period (e.g. the i-th row processing cycle shown in FIGS. 18A-18D), the waveform of a voltage signal applied to the top gate terminal TG is asymmetrical with respect to 0 V (GND level). The effective voltage applied to the top gate terminal TG is Vte shown in FIG. 18A, which is greatly biased to the low level side (negative voltage side). Similarly, a high level voltage (e.g. +10 V) is applied for a short time to the bottom gate terminal BG, and a low level voltage (GND level) is applied thereto during the other period. Thus, the waveform of a voltage signal applied to the bottom gate terminal TG is asymmetrical with respect to 0 V (GND level). The effective voltage applied to the bottom gate terminal BG is Vbe shown in FIG. 18B, which is greatly biased to the high level side (positive voltage side).
If, in a photosensor having a thin film transistor structure, such a biased voltage is continuously applied to each gate terminal in a state in which light is applied to the sensor, for example, charge (positive holes or electrons) is trapped in each gate electrode, thereby degrading the element characteristics of the photosensor and hence changing its sensitivity. As a result, the reliability of the photosensor reduces.
(2) Further, when a photosensor system using the aforementioned photosensors is used in various places or used to pick up images of various types of subjects, the subjects may have different brightnesses and these brightnesses may be varied depending upon the states of their environments. In order to accurately read images of various types of subjects in various environments, it is necessary to set the sensitivity of the photosensor to a value suitable for each subject and/or environment, and to read its image at the set sensitivity. The sensitivity of the photosensor is determined from, for example, the amount of charge accumulated within the charge accumulating period, i.e. corresponding to the total amount of incident light during the period. Accordingly, the sensitivity can be adjusted by adjusting the charge accumulating period. This being so, even when the effective voltage applied to each gate terminal is set at an optimal value, if the charge accumulating period is changed to a value appropriate for each environment, the effective voltage applied to each gate terminal will inevitably change and deviate from the optimal value. This changes, for example, the aforementioned sensitivity characteristic, thereby making it difficult to secure sufficient reliability of the image reading apparatus.